The Government says it is up to employers to determine if hazards such as asbestos are present in a workplace before work begins.
Contractors Liam Milner and Neil Silcock say they were exposed to asbestos on the roof of Christchurch Hospital and a co-worker was covered in the material while working at a house as part of the city's rebuild - but nothing was done about it.
Employment law specialist David Beck is representing the contractors who he says told their employer, Goleman Ltd, about asbestos falling onto a co-worker at a Cashmere house in October 2012, but were ignored.
Mr Beck said on Monday that government regulations are not being enforced properly.
"The regulatory framework of the Ministry of Business and Innovation is just hopelessly under-resourced and has not been very active in this area around the rebuild, particularly around asbestos-related claims.
"They don't appear to have taken a very rigid approach to this."
The Ministry of Business, Innovation and Employment said it is up to employers to identify such hazards before any work starts.
The ministry's Canterbury rebuild health and safety programme director, Kathryn Heiler, says if work has already started when asbestos concerns are raised, then the work must stop and tests must be done to establish whether the substance is present or not.
Fletcher Earthquake Recovery says it is investigating a workplace incident involving a contractor in the Canterbury home repair programme, but won't comment further until the investigation is finished.
Continued inhalation of asbestos particles can cause terminal lung disease.